FIGS. 11(a) to 11(c) are cross-sectional views illustrating process steps in a conventional method for fabricating a waveguide type photodiode.
In these figures, reference numeral 1 designates an n type (hereinafter referred to as "n-") InP substrate (wafer) having a surface orientation of (001). An n-InP lower cladding layer 5 having a thickness of 0.5 .mu.m and a carrier concentration of 1.times.10.sup.18 cm.sup.-3 is disposed on the substrate 1. An n-InGaAsP guide layer 6 having a thickness of 0.8 .mu.m and a carrier concentration of 1.times.10.sup.18 cm.sup.-3 is disposed on the lower cladding layer 5, and its composition ratio is 1.4 .mu.m. An undoped InGaAs absorption layer 7 having a thickness of 0.6 .mu.m is disposed on the guide layer 6. A p type (hereinafter referred to as "p-") InGaAsP guide layer 8 having a thickness of 0.8 .mu.m, a carrier concentration of 1.times.10.sup.18 cm.sup.-3, and an optical absorption wavelength of 1.4 .mu.m is disposed on the absorption layer 7. A p-InP upper cladding layer 25 having a thickness of 2 .mu.m and a carrier concentration of 1.times.10.sup.18 cm.sup.-3 is disposed on the guide layer 8. A p-InGaAs contact layer 10a having a thickness of 0.25 .mu.m and a carrier concentration of 1.times.10.sup.19 cm.sup.-3 is disposed on the upper cladding layer 25. Reference numeral 40 designates a mask for selective growth, comprising an insulating film of, for example, SiO.sub.2 (hereinafter referred to as a selective growth mask). Reference numeral 3 designates an Fe (iron) doped InP window layer having a thickness of 4 .mu.m and a dopant (Fe) concentration of 4.times.10.sup.16 cm.sup.-3.
FIG. 12 is a plan view, showing a main step of the fabrication method. In FIG. 12, the same reference numerals as those shown in FIGS. 11(a)-11(c) designate the same or corresponding parts. The cross-sectional views of FIGS. 11(a)-11(c) are taken along a line 11--11 of FIG. 12.
A description is given of the method of fabricating the conventional photodiode. Initially, as shown in FIG. 11(a), the n-InP cladding layer 5, the n-InGaAsP guide layer 6, the InGaAs absorption layer 7, the p-InGaAsP guide layer 8, the p-InP layer 25, and the p-InGaAs contact layer 10a are successively grown on the (001) surface of the n-InP substrate (wafer) 1 by MOCVD (Metal Organic Chemical Vapor Deposition).
Next, as shown in FIG. 11(b), over the entire surface of the grown semiconductor layers, an SiO.sub.2 film (not shown) is deposited by sputtering and patterned by photolithography, forming an SiO.sub.2 selective growth mask 40. The mask shape viewed from the top of the structure is, as shown in FIG. 12, a rectangular area having a width of 10 .mu.m and a length of 20 .mu.m. Using the selective growth mask 40 as a mask for etching, the grown semiconductor layers are etched to a depth of 4.5 .mu.m using a bromine based etchant. Since this etching is isotropic, each semiconductor layer is subjected to side-etching. As a result of the side etching, the area of the InGaAs absorption layer 7, viewed from the top of the structure, becomes approximately 4.times.14 .mu.m.sup.2.
Thereafter, as shown in FIG. 11(c), using the selective growth mask 40, the Fe-doped InP window layer 3 is selectively grown on the side surfaces of the structure formed by the above-mentioned etching, using MOCVD, followed by removal of the SiO.sub.2 selective growth mask 40.
Thereafter, a p side electrode and an n side electrode (not shown) are formed on the surface of the contact layer 10a and the rear surface of the substrate 1, respectively, completing a photodiode.
The operating principle of the photodiode so fabricated will be described. Light incident on a facet of the photodiode perpendicular to the surface of the substrate 1 enters the InGaAs absorption layer 7 through the Fe-doped InP window layer 3, and the light is absorbed in the layer 7. Electrons and holes generated by the optical absorption in the layer 7 are immediately swept by an electric field that is generated by a reverse bias applied to the photodiode, and are taken out as light signals. By decreasing the thickness of the InGaAs absorption layer 7, it is possible to make the photodiode correspond to high frequencies of 40 GHz or more. The InGaAsP guide layers 6 and 8, disposed across the InGaAs absorption layer 7, confine the incident light in a region near the edge of the absorption layer 7 by reflecting the light in the direction perpendicular to the substrate 1, so that the incident light is efficiently transmitted deep into the device, thereby improving the light absorption efficiency.
In a photodiode as shown in FIG. 11(a), where the layers 5 to 10a are simply grown on the substrate 1, a pn junction between the guide layers 6 and 8, through the light absorption layer 7, is exposed at a facet of the device. In this photodiode, since the absorption layer 7 is exposed at the facet, many surface states are generated in a portion of the absorption layer 7 in the vicinity of the facet, and incident light is absorbed by these surface states during the operation of the photodiode. As a result, dark current increases, and a portion in which light is absorbed generates heat. After many hours, dislocations occur due to that portion, resulting in optical deterioration of the photodiode. In addition, since the facet of the photodiode is exposed, the edge of the absorption layer 7 is adversely affected by exterior environments, such as oxidation, leading to deterioration. Consequently, it is impossible to provide a photodiode excellent in reliability.
In order to solve the above-mentioned problem, in the conventional photodiode, as shown in FIG. 11(c), the window layer 3 having a band gap energy larger than that of the guide layers 6 and 8 is formed at the facet so that the light responsive part of the photodiode has a window structure. Since the window structure prevents the edge of the absorption layer 7 from being exposed, surface states are reduced, avoiding optical deterioration of the photodiode and deterioration of the absorption layer 7 due to exterior environments. The reason why the window layer 3 is doped with iron is to increase the resistance of the window layer 3 for preventing current leakage. In place of iron, a transition metal, such as cobalt, vanadium, or titanium, may be used. Alternatively, instead of doping with iron, an undoped window layer having a high resistance may be formed.
However, the conventional method of fabricating a light detecting device, such as a photodiode mentioned above, has the following drawbacks. Since the window layer is formed by etching portions of semiconductor layers successively grown on the substrate and then regrowing the window layer in a region produced by the etching, an interface of the grown semiconductor layers and the regrown window layer (hereinafter referred to as a regrowth interface) is exposed, and new surface states are produced on the regrowth interface because the regrowth interface is exposed and contaminated. When surface states are present on a portion of the absorption layer, a depletion region, adjacent to the regrowth interface, light is absorbed by the surface states and dark current is generated during operation. For example, dark current increases by approximately 200 nA. Such dark current usually increases in proportion to the area of the regrowth interface.